The present invention finds application in connection with thin silicon plates or wafers formed to support a multiplicity of monolithically integrated data processor circuits. More particularly, the invention is directed to the production of circuits formed on silicon wafers to include conductive pads or films formed on at least one edge thereof, with the remaining portion of that edge being insulated from the silicon material. The wafers may be stacked and adhesively bonded to form a data processor module that can be bump bonded to an input source, e.g., an infrared detector array, connected to the module along the edge portions thereof. Conductive pads formed on the edge portions of the wafers opposite to the input source can be similarly bump bonded to an array of connector contacts such as a pin grid array or a printed circuit board. A plurality of modules can be joined together and interconnected electrically to form an assembly, e.g. an infrared detector processor assembly.
Though silicon wafers formed in accordance with the present invention may have application in a variety of different areas, the present invention is described in connection with the production of modules for space-borne infrared detection systems, wherein particular requirements with respect to space, size and ability to operate in extremely low temperature environments present criteria for which the present invention has particular advantages. In view of the space and weight limitations imposed on objects designed to be placed in space there is a particular need to develop processing modules and connecting devices that can reliably operate without imposing substantial weight or space penalties on the payload.
In order to provide accurate detection and resolution of objects characterized by an infrared signature, it is typically necessary to employ detection systems having a large number of discrete detector elements. The detector elements are interconnected to form a detector array, which in turn is connected to circuitry to allow the array to "scan" or "stare" over a substantial field of view. Accordingly, each of the detector elements must be electrically connected to processing circuitry in a manner wherein signals from adjacent detector elements may be separately detected and processed. Because the detector elements are small and very closely spaced, e.g., 0.003 inches center to center spacing, the circuitry for processing signals from the detector elements must conform to similar size and space limitations. Many conventional schemes for connecting detector elements to the processing circuitry are unsuitable to provide the required isolation, and reliability. Moreover, production techniques for connecting the individual detector elements to dedicated processing circuitry are typically expensive, tedious and characterized by a low degree of reliability.
The technique for connecting infrared detector elements and the dedicated processing circuitry requires that the inputs and outputs of the processor circuits be electrically isolated. When the processor circuits are formed on stacked silicon wafers, it is necessary to isolate the conductive edge portion from the active circuitry formed on the silicon wafer (to prevent undesired communication between the inputs or outputs and the processor circuit). Previous disclosures modify the vertical edge portions of the semiconductor wafers after the wafer has been fabricated and the plates are cut therefrom, to form a non-conductive region on the edges of the finished wafers to provide this isolation. For example, U.S. Pat. No. 4,551,629, to Clark, teaches that stacked wafer, i.e. silicon integrated circuits, may be connected to a detector array by selectively etching between metallized edge portions of the semiconductor wafers and then refilling the etch removed material with an insulator. The technique for selectively etching and backfilling edge portions of such small, thin wafers is tedious, expensive and difficult.
U.S. Pat. No. 4,618,763 to Schmitz, assigned to the common assignee, discloses that a wafer construction formed of epitaxially grown silicon formed on an insulator sapphire base. The silicon is removed from the sapphire near the edge portion to provide an insulator substrate for isolated conductive film leads. Though feasible, this construction utilizes integrated circuit technology that is less practiced than that of using a bulk silicon substrate. Additionally, because the sapphire substrate is harder and more difficult to produce than silicon, it is more difficult to grind the wafer to the required thinness necessary to form a high density processor channel module and it is more expensive.
The present invention is directed to a processor construction particularly suited for high density environments, where conductive end and edge portions may be isolated from the silicon material by the formation of insulator moats constructed in the course of the wafer fabrication process. The insulator moats are formed in the silicon wafer which, after appropriate thinning and sizing provides the desired insulator substrate end and edge portions of the wafers. Various techniques are disclosed for forming the insulator moats, and isolating the silicon from adjacent wafers in a wafer stack.